1. Field of the Invention
The present invention relates to the preparation of chemically stable, dry-flow, low compact, dust free, soluble granules of phosphoroamidothioate using a substantially dry granulation process including an agitative balling process. This dry granulation process preferably produces spherical granules of phosphoroamidothioate without using substantial amounts of water, liquid, solvent or binder.
2. Description of Related Art
In recent years, agricultural chemicals have been most preferably formulated in the form of dusts, wettable powders, soluble powders, emulsifiable concentrates, soluble liquid/concentrates, granules, coated granules, water dispersible granules, suspension concentrates, and solutions. Occasionally, when dusts are produced by absorbing or mixing active ingredients with a finely divided inert carrier material, for example China Clay or the like, drift problems occur. With wettable powders and soluble powders the problems faced at the time of dilution are not only drift, but the final disposal of containers, for dust particles tend to stick to sides of the containers. The left over materials within the containers pose great problems to the environment, operators and users.
Although dusts are undesirable because of airborne contamination and handling difficulties, liquid spray formulations have not provided an acceptable alternative, for they involve solvents and packaging expenses, along with container disposal requirements that detract from their commercial desirability.
Water dispersible granules produced by fluidized bed spray dryers overcome the problems associated with wettable powders and soluble powders, but have high processing costs and require high value capital investment, as well as requiring highly skilled staff. These problems impose a significant barrier in widening the market acceptance of these compounds.
Certain phosphoroamidothioates and phosphoroamidodithioates, collectively referred to as phosphoroamidothioates, are known to have excellent insecticidal activity against a variety of insects and in a variety of environments. Acephate, one of the important commercial insecticides within this class of compounds, is a systemic and contact insecticide of moderate persistence with residual activity lasting about 10–15 days. It is effective against a wide range of aphids, leaf-miners, lepidopterous, larvae, sawflies and thrips and it is also a non-phytotoxic on many crop plants.
Phosphoroamidothioate containing pellets have been proposed in the past, but difficulties have been encountered in pelletizing acephate technical, the preferred insecticide within the class of phosphoroamidothioates. Attempts to manufacture acephate technical pellets from acephate technical powders have been proposed and have several disadvantages. As disclosed below, prior extrusion processes include the addition of costly surfactants, the combination of phosphoroamidothioate with a second active ingredient, or the creation of a mixture of the active ingredient with a solvent in an amount of from 3–25% by weight before extrusion.
One formulation of acephate presently in use is acephate 75% soluble powder having acephate active ingredient (a.i.) 75% (w/w), surfactant 1 to 2% (w/w), inert filler (precipitated silica) to make 100% (w/w). Acephate 75% soluble powder poses several problems including the production of dust, low pourability of the powder, high transportation costs, high capital manufacturing investment, measurement difficulties, difficulties in packing, material disposal, handling problems, high risk of caking and others.
A previous process for preparing pellets comprising insecticidal N-hydrocarboyl phosphoroamidothioates and/or phosphoroamidodithioates includes the extrusion of a solid composition. The concentration of the active ingredient in the pellets prepared is in the range of about 2 to 80% a.i., with the most likely concentration of 70% a.i. The process comprises (i) forming an extrudable mixture comprising the active ingredient; (ii) forming said pellets by extrusion of the mixture and cutting the extrudate. Another similar method comprises (i) forming a suspension or solution containing said active ingredient and a dispersant, wetting agent and/or surfactant; (ii) evaporating the solvent from said solution or dispersion; (iii) dividing the remaining solids into particles; and (iv) forming the resultant mixture into pellets. These processes experience several limitations and difficulties. First, the amount of active ingredient is in the range of about 2 to 80%, preferable being 70% so the process does not produce pellets having a concentration of active ingredient above 80%. Second, the process envisions two active ingredients namely N-hydrocarboyl phosphoroamidothioates and phosphoroamidodithioates in a single product. The selection of the combination of ingredients is important so that the product does not deteriorate in a few days to a few months. Third, the process involves forming a suspension or solution of the active ingredient and a dispersant, wetting agent and/or surfactant, and evaporating the solvent which are complicated processes and make the manufacturing process cumbersome and uneconomical.
In another previously used process for pelletizing insecticidal N-hydrocarboyl phosphoroamidothioates and phosphoroamidodithioates, the insecticidal ingredient in the pellets formed is in the range of about 50 to 95% preferably about 90%. This process involved mixing technical grade insecticide in a dry form with one or a mixture of surfactants (10% by weight of total dry composition) preferably containing an inert diluents ammonium sulfate (<40% of total pellet composition preferably 2% or less); solid additive (up to about 40% anhydrous Magnesium sulphate) and small amounts of deodorants and antifoam agents. The dry ingredients are ground to powdered form. Currently available pelletized acephate, known commercially as ORTHENE, requires the use of an anticaking agent, such as fumed silica. This process also has various limitations and difficulties. The amount of insecticidal ingredient present in these pellets is in the range of about 50 to 95%, preferably about only 90%. The final product may also contain inert diluents ammonium sulfate (<40% of total pellet composition preferably 2% or less) by weight of the total pellet composition. The presence of this agent is not desirable in the final product as it adds to the cost and imparts hardness to the granule when the active ingredient in the granule is in low concentration. The product may also include up to about 40% anhydrous magnesium sulphate, preferably 2% or less by weight of the total pellet composition, as a dehydrating agent to absorb trace amounts of the water present in the pellets to prevent hydrolysis of the insecticide. The presence of this agent is not desirable in the final product because it adds insoluble matter into the granules which causes problems in the application of the product.
In yet another previous process for forming pelletized insecticidal N-Hydrocarboyl phosphoroamidothioates and phosphoroamidodithioates, extrusion of a damp sandy loam of a dry mixture of the active ingredient and a solvent is formed. Several limitations and difficulties are associated with this process. First, the process involves mixing of active ingredient in a solvent to form a consistency of damp sandy loam and then pellets are formed by extrusion. Second, the process involves passing the mixture of active ingredient and the solvent in molten form through an orifice to form molten pellet-sized drops of said active ingredient and solidifying the said drops. These steps increase the processing costs of making the pellets because of the addition of costly solvents and the additional drying steps to remove the solvent and/or moisture.
One rapidly expanding field used in a variety of chemical processes is an agitative balling process for the formation of granules. Current processes used for making granules using the agitative balling process produce agglomerations and the resulting granules by admixing fine powder with binders, water or other types of liquids. The addition of binders, waters and other liquids increases the processing costs in formulating the granules.
Because of the problems associated with producing granular forms of phosphoroamidothioates, such as the preferred acephate, there is a need in the art for a process for preparing chemically stable, dry flow, low compact, dust free, insecticidally active soluble granules of phosphoroamidothioate which are useful from a practical stand point, as well as for a low cost, practical manufacturing technique which can be practiced on a commercial scale without requiring expensive additives or solvents.